The present invention relates to an apparatus and method for automatically wave soldering workpieces such as printed circuit boards under an insert gas atmosphere.
A conventional automatic wave soldering apparatus typically includes a flux applicator, a preheater, a solder station and a cooling station subsequently arranged to process printed circuit boards. While the printed circuit boards are transported by a conveyor with their side edges supported by gripping fingers, flux is first applied by contacting each circuit board with a foam of flux. Alternatively, flux may be applied to the circuit board by spraying. The fluxed board is preheated by the preheater in order to evaporate excess flux solvent and prevent sudden extreme temperature changes or xe2x80x9cheat shockxe2x80x9d to the board. The circuit board is then contacted with multiple waves of molten solder. Typically, the solder station includes a relatively narrow nozzle to produce a turbulent wave. This turbulent wave enables the molten solder to fill the gap between leads of electronic components and through holes in the circuit board. However, solder bridges, icicles or excess solder deposits are apt to remain on the underside of the circuit board. To remove such undesirable bridges, a relatively wide nozzle is provided downstream of the narrow nozzle to produce a smooth turbulent free solder wave through which the circuit board passes.
The electronic components are heated to a high temperature when current flows therethrough. On the other hand, the electronic components are cooled to a room temperature in an inoperative state. This thermal cycling causes expansion and contraction of the board as metallic solder and the plastic printed circuit board have different coefficients of thermal expansion. The resulting mechanical stress causes the solder to fracture or peel.
To avoid such solder fracture or fatigue, attempts have been made to deposit a relatively large amount of solder on selected regions of a board to be soldered, as disclosed by Japanese laid-open patent application No. 9-283912. A nozzle is arranged in a solder bath and has a fixed front guide and an adjustable rear guide. A baffle plate is attached to the nozzle near the rear guide. The rear guide is inclined downwards in the direction of movement of a board to thereby form a step between the baffle plate and the rear guide. The molten solder flows at a relatively fast rate after it flows over the baffle plate. This enables a relatively large amount of molten solder to be deposited on selected regions of the circuit board to be wave soldered. However, application of such a large amount of molten solder is apt to form solder bridges, particularly in case that electronic components are packaged with high density. It is, therefore, necessary to adjust the flow rate of molten solder depending on the density of packaged components. This adjustment is made manually, or automatically as disclosed by Japanese laid-open patent application No. 9-293959.
A tin-lead solder has superior wetting characteristics and is conventionally used with RA flux or RMA flux. The use of RA flux minimizes or eliminates the occurrence of bridging regardless of whether solder is applied to high density circuit boards or a large amount of solder is applied. However, RA flux residues are corrosive or hydrolyze to corrosive constituents in the presence of moisture. Those flux residues must therefore be rinsed with chlorine solvent, fluorine solvent, hydrocarbon solvent, terpene solvent or other solvents. All of those solvents are considered to be environmental pollutants. The RMA flux is less corrosive and requires no cleaning after soldering. However, the RMA flux is less active than the RA flux and tends to form solder bridges. The use of a lead-free solder also forms solder bridges since the lead-free solder exhibits a high degree of surface tension and a low degree of wettability.
It has been found that formation of such solder bridges can be avoided if soldering occurs in a substantially oxygen-free atmosphere. Typically, an inert gas is directed to the point, known as xe2x80x9cpeel backxe2x80x9d region, at which a printed circuit board exits from a solder wave. To reduce oxygen content in that region, an inert gas nozzle is placed as close to the peel back region as possible. However, the single use of the inert gas nozzle is not satisfactory since the concentration of oxygen in the peel back region are apt to fluctuate.
Accordingly, it is an object of the present invention to provide an automatic wave soldering apparatus and method which can provide a steady supply of a reduced oxygen atmosphere near the point at which circuit boards exit from a solder wave.
According to one aspect of the present invention, there is provided an apparatus for wave soldering a workpiece such as a printed circuit board, which comprises a solder reservoir adapted to contain molten solder, and a solder nozzle extending up above the molten solder and arranged to provide a substantially turbulent free solder wave under the workpiece. A tray is pivotably mounted to the downstream side of the solder nozzle. The tray is angularly moved to vary or adjust the rate of flow of the molten solder after the molten solder is pumped up through the solder nozzle. A shroud or enclosure is mounted adjacent to and associated with the tray to define a contained space into which an inert or non-oxidizing gas is supplied to provide an inert or non-oxidizing gas atmosphere. The shroud includes an adjustable canopy extending over a portion of the tray and selectively moved toward and away from the solder nozzle in a direction substantially parallel to the path in which the workpiece is moved. The canopy is adjustably positioned in response to angular position of the tray to ensure that the workpiece exits from the solder wave within the inert gas atmosphere.
The point at which the workpiece exits from the solder wave is referred to in the art as a xe2x80x9cpeel back regionxe2x80x9d. This peel back region is displaced depending upon the rate of flow of the molten solder. The peel back region should be blanketed with an inert gas atmosphere in all cases; otherwise, solder bridges or icicles are likely to occur. According to the present invention, the tray is angularly moved downwards to increase the rate of flow of the molten solder when a relatively large amount of solder deposits are required. At this time, a peel back region is defined at near the point where the tray is attached to the solder nozzle. It is therefore necessary to position the canopy as close to that point as possible so as to ensure that the peel back region is blanketed with an inert gas atmosphere. As stated above, the canopy extends substantially parallel to and below the path in which the workpiece is moved. This arrangement allows the canopy to position as close to the peel back region as possible. Such close position aids in reducing oxygen content near the peel back region. When a relatively small amount of solder deposits are required, the tray is angularly moved to a horizontal position or slightly upwardly inclined position to decrease the rate of flow of the molten solder. At this time, a peel back region is displaced away from the point at which the tray is attached to the nozzle. To this end, the canopy is moved in a direction away from the nozzle and positioned near the peel back region. This movement of the canopy will not interfere with the position of the tray as the canopy is movable along the path in which the workpiece is moved.
In a preferred embodiment, a baffle may be located at the downstream end of the solder nozzle and associated with the tray to form a step when the tray is oriented in a downwardly inclined position. By this arrangement, the molten solder is caused to drop when it flows over the baffle. This enables the molten solder to flow at a relatively fast rate. To allow angular movement of the tray, a bracket may be secured to the solder nozzle, and a crank may be disposed between the bracket and the tray. The crank may be connected to a motor and driven to cause the tray to rotate in a vertical plane.
In a preferred embodiment, the shroud may include a mount secured to the solder reservoir and shaped to slidably support the canopy thereon. A second solder nozzle may be located upstream of the solder nozzle to provide a turbulent wave under the workpiece. A weir member may be mounted adjacent to the tray and cooperate with the tray to form an opening through which the molten solder flows back to the solder reservoir. The weir member may be selectively rotated toward and away from the tray to vary the degree of opening of the opening.
According to another aspect of the present invention, there is provided a method of wave soldering a workpiece, which comprises the steps of providing a wave soldering apparatus including a solder reservoir within which molten solder is contained, a solder nozzle disposed in the solder reservoir, a tray pivotably mounted to the solder nozzle, and a shroud including an adjustable canopy and associated with the tray to form a contained space, forcing the molten solder through the solder nozzle to generate a substantially turbulent free solder wave under the workpiece while the workpiece is moved in a predetermined path, angularly moving the tray to vary the rate of flow of the molten solder, providing an inert gas atmosphere within the contained space, and adjusting position of the canopy in response to angular position of the tray to ensure that the workpiece exits from the solder wave within the inert gas atmosphere.
Depending on angular position of the tray, the canopy is moved toward and away from the solder nozzle in a direction substantially parallel to the path in which the workpiece is moved. The tray is oriented in a downwardly inclined position so as to increase the flow rate of the molten solder when a relatively large amount of solder deposits are required. When a relatively small amount of solder deposits are required, the tray is oriented in a horizontal position or slightly upwardly inclined position to decrease the flow rate of the molten solder.